Conventional AGC systems are generally adequate to provide a nearly constant amplitude output signal as a sensed input signal varies in amplitude over a given range. However, when the input signal ceases, typical AGC action increases system gain to its maximum. This action gives rise to two problems. First, the level of background noise is greatly enhanced under maximum gain conditions. Second, should a large audio input signal arrive while the system is at maximum gain, an annoying large amplitude voice burst will be developed.
To overcome the foregoing problems with AGC, some audio systems now incorporate ALC. As with AGC systems, an ALC system operating on audio signals allows system gain to vary freely in the presence of the desired audio signal. However, ALC systems generally include a voice detection circuit that senses valid incoming voice signals and attempts to hold system gain to its most recent value when the incoming voice signal terminates. Theoretically, input noise which arrives in the absence of voice will not be greatly amplified, and the next voice signal to arrive will initially be amplified at the same level of gain which the last voice signal received. However, other problems with voice processing systems can fool conventional ALC systems and lead to reduced intelligibility. Examples of such problems are discussed with reference to FIGS. 1, 2 and 3.
Referring first to FIG. 1, a typical ALC system is shown as including a variable gain amplifier 10, a gain adjust block 12 for sensing the output of the amplifier 10 and varying its gain, and a voice detector 14. This illustrated system is similar to a conventional AGC system, except for the inclusion of the voice detector 14. Suffice it to say, the voice detector 14 looks for the presence of voice signals above a threshold and holds the gain adjust block 12 at its last valid gain value in the absence of voice. When a new voice signal is detected, the gain adjust block is allowed to adjust gain level according to the amplitude of the new voice signal.
Typically, the attack time (the time required for the gain adjust block to change from a high gain to a low gain condition) is chosen to be very fast. Its release time (the time required for the gain adjust block to change from a low gain to a high gain condition) is chosen so that when a new, lower amplitude voice source is received, the first few words or syllables will not go unheard.
In a typical application, the audio input to the ALC system shown in FIG. 1 is adapted to be switched between voice signals carried by different telephone lines which have different loss characteristics. Since these losses are somewhat unpredictable and vary with time, the ALC system should be capable of operating over a 35 db dynamic range.
Previous attempts to implement an ALC system have been only partially successful. Some of the problems encountered include mistaking noise for voice signals and failing to deliver intelligible speech. To further illustrate the latter problem, reference is made to FIGS. 2 and 3.
In FIG. 2, a speech pattern is shown which exemplifies a typical spoken syllable which is well within the illustrated 35 db dynamic range. Only the shaded voice peaks 16 and 18 extend beyond the threshold of the voice detector to pass an appropriate control signal to the gain adjust block 12. Thus, the gain adjust block 12 is permitted to respond to the peak 16 to adjust gain accordingly. However, since the voice lulls 20, 22 and 24 are below the voice detector's threshold, the gain adjust block holds the gain constant at the level previously established in response to the peak 16. This is a desired effect for voice intelligibility.
FIG. 3 shows another voice signal in which voice lulls 26, 28 and 30, as well as peaks 32 and 34, exceed the threshold of the voice detector. Accordingly, the gain adjust block 12 is enabled to change system gain during the lulls and the peaks. Thus, as the speech envelope decays, the gain adjust block increases gain in an attempt to maintain a constant output level. Not only does intelligibility suffer under such a condition, but the signal to noise ratio is degraded during the voice lulls. Words having a substantial low frequency content are particularly susceptible of being improperly amplified so that an increase in the ambient noise level is perceived.
To overcome the increase in noise which occurs during lulls of the type shown in FIG. 3, the decay rate of the gain adjust block may be reduced. However, this tends to render the ALC system insensitive to rapid amplitude variations which occur when the audio input is switched to a different voice source having a relatively higher loss characteristic. The first few syllables or words from the new voice source may go unheard because a longer decay rate prohibits a rapid adjustment in gain. Compromises between these conflicting requirements generally result in unsatisfactory intelligibility.
Another problem frequently associated with prior ALC systems is their inability to accurately hold constant or freeze the operation of the gain adjust block during intervals when voice is absent to prevent an unwanted increase in system gain. Typically, ALC systems attempt to freeze the gain level by holding constant a gain control voltage on a capacitor. This approach is satisfactory for short periods of time, but the charge on the capacitor eventually leaks off through various circuit elements, thereby permitting system gain to increse when it should be held constant.
Accordingly, it is a general object to the invention to provide an improved ALC system.
It is a more specific object of the invention to provide an ALC system which exhibits improved voice intelligibility.
It is another object of the invention to provide an ALC system which prevents the system gain from increasing for an indefinite duration in the absence of voice.
It is a further object of the invention to provide an ALC system which provides a high degree of noise and transient rejection to avoid changing the gain level in the presence of various types of noise.